Photosensitive compounds or adhesives are commonly used in bonding object surfaces together or for filling openings and cavities in an object surface. They are cured by exposure to radiation energy, such as UV with a wavelength between 300 to 400 nm or blue light with a wavelength between 400 to 500 nm. Medical curing light device are commonly used in dentistry, endoscopies, and plastic surgeries. In the field of dentistry, curable adhesives and dental curing apparatus are common practice in restoration and cosmetic procedures using restorative materials, dental sealants and orthodontic adhesives to bond brackets to the surfaces of teeth. Curing light is also widely used in device, component and circuit board packaging using photo-initiator activated composites to bond two different surfaces or protect components.
Traditionally, curing light apparatus are implemented with bulk lamps such as tungsten-halogen lamps coupled into fiber optic waveguide that delivers light to expose area of adhesives need to be cured. Recent advances in light emitting diodes (LEDs) technologies have enabled a new class of curing light apparatus with smaller size, longer lifetime and lower cost by semiconductor light emitting chips.
LEDs emit light at selected wavelengths of absorption band of photo-initiators that start the curing process of curable adhesives. Typical wavelength for dental curing is in the range of 400-500 nm. It is highly desirable to have high optical density impinged on the curable adhesives to activate the photo-initiators that allow a quick curing time of between 2 to 10 seconds and a deeper curing depth of between 2 to 6 millimeters. Typical ranges of optical density for a desirable 4 to 5 millimeters curing depth and less than 10 seconds curing time are above 1000 mW/cm2. In dental applications, such intensity is exposed to the curing area, typically in the range of 2 to 6 mm dimension, limited by the cavity and bracket size.
There have been two approaches in the selection of LEDs to achieve such high intensity, namely single high power LEDs or multiple standard single diode LEDs. High power LEDs integrates multiple LED chips in a single package such as LEDs made by Lumiled's Luxeon product lines that generate optical power as high as 800 mW. Standard single chip LEDs generates optical power below 150 mW. Typical arrangements of more than five LEDs are required to deliver equivalent power at the curing site. Other critical elements of efficient curing are the light delivering system and working distance of the curing apparatus from the curing object for efficient cure.
U.S. Pat. No. 6,611,110 describes an apparatus using light guides to deliver curing light from a single LED to the curing site. The light guide reduces the deliverable curing light efficiency due to optic coupling, transmission, and diffraction losses from light guide with a typical total efficiency of below 30%. A higher power LED can compensate the loss. Additional use of lens such as total internal reflection (TIR) lens as described in U.S. Pat. No. 6,692,251 can improve the power density. However, they introduce higher cost and more cumbersome system. Additionally, it has been shown that autoclaving the light guide to sterilize the apparatus can reduce the transmission performance of the light guide making them costly to replace.
U.S. Pat No. 20030133203 describes an apparatus using a bulk aspheric lens to directly focus curing light from a single LED to the curing site. The aspheric lens is molded glass or plastic lens. The benefit of such implementation is a reduced size and cost compared to using of light guide. However, a high power LED is highly non-directional typically following a Lambertian radiation pattern with radiation angles above 120 degrees at half of its maximum intensity. Combined with a source chip size of typically 3 millimeter, the LED radiation incurs collection loss through the aspheric lens and diffracts quickly to lose its intensity due to limited collection angles that aspheric lens offer, which is typically less than 70 degrees. Aspheric lenses with short focal length to collect light from LED source are also thick with aspect ratio of diameter to thickness close to one enlarging the size of the apparatus as well. Working distances of such devices are typically limited to a short distance within a few millimeters. In addition, sterilizing tubes to protect the lens entrance will significantly reduce radiation due to optical diffractions.
A need exists, therefore, for improved LED curing apparatus that provide efficient light delivery to the curing site at high optical intensity with low cost particularly in the dental applications.